1. Field of the Invention
The present invention relates, in general, to timing based servo patterns that are laterally positioned on a linear data storage tape and, more particularly, to verification and measurement of nonlinearity of Position Error Signals in timing based servo patterns using differential nonlinearity.
2. Relevant Background
High density recording of digital information on magnetic tape continues to place more and more information on extremely narrow tracks. As technology progresses, it is reasonable to expect that thousands of tracks will exist on a single tape. Currently, tracks are on the order of 20 microns wide, yet to achieve storage densities on a tape cartridge in the order of 1-10 terra bytes (TB), track widths in the order of 5 to 1 microns will be necessary. Such high density recording accordingly requires the magnetic head tracking to be very accurate.
Servo signals developed from servo stripes written into the magnetic tape have long provided a means to accurately position the read/write heads over a track. Time-Based servo bands or patterns, which are written to the tape during the manufacturing process, are used as a reference to position all data tracks for the life of the cartridge. The servo pattern is comprised of a series of stripes that repeat down the length of the tape. The lateral positioning of a tape head in response to timing measurements based on the servo bands is well known in the art. Essentially, the read/write head is positioned based on timing of signals received from separate transducer heads of a timing based servo system that detects the servo stripes. In an actual drive, the recording head is mounted on a movable actuator and its position is constantly being monitored and corrected to compensate for lateral tape motion. The actuator is adjusted using a Position Error Signal (“PES”), which is the difference between the measured position computed from the servo pattern and the desired position of the head. PES is, therefore, essentially an indication of the position of the head as a function of the true or actual position of the head. Ideally, an insignificant (minimal) PES would signify that the indication where the head is corresponds to its actual location. Thus, when the head is properly registered, meaning the PES is sufficiently small, data tracks are written to the tape accurately. However, as the density of the tracks increase driving the tracks narrower and narrower, the need to accurately position the read/write head and minimize any PES is magnified. Certainly inaccuracies in the placement of the servo stripes with respect to one another becomes a significant factor in PES as does the consistent shape of each stripe.
The time-based servo stripe pattern is factory written as part of the tape build process using a servo write head. The servo write head is typically a mirror image of a single servo frame with two non-parallel write gaps on the recording surface. A pair of stripes are written simultaneously by pulsing the write head with current while transporting the tape on a precision tape deck. By controlling tape speed and the timing of the write pulses, the pattern is repeatedly stamped down the tape with precise control over fame to frame spacing. Unfortunately the process is not perfect and significant imperfections occur in each stamping.
Noise in a timing based servo systems for tracks larger than 20 microns is well known and has been studied extensively and even perfectly tracking a track on a tape will produce a certain amount of noise. Noise of this type is well known and there are a number of techniques for minimizing and compensating for the PES that is associated with such noise. A significant limitation of this type of analysis is that the servo bands, regardless of their shape and orientation, are typically assumed to produce a PES that is linearly perfect. Techniques exist that can address the linearity of the width of each stripe of the servo stripe series and determine how the PES varies as the cross track position along the band varies. Thus, when the curvature of the servo stripe is known, these analysis techniques can address such factors, but their applicability falters when the curvature of the servo stripes is unknown.
Determining the variations in the gaps in the servo write head and shape of the servo stripes on the tape is problematic. Historically, the low density of data, and thus the relatively thick nature of the data tracks, has been such that the PES introduced due to characteristics of the shape of the servo stripe has been insignificant or of minimal consequence in comparison to other sources of PES such as lateral tape motion or frame spacing error. However, as track density increases, the PES introduced by nonlinear characteristics, such as those induced by the shape of the servo stripes, becomes more and more problematic. And while current tape manufacturers that imprint the servo stripes on the tape have taken steps to limit the curvature of the stripes and make the shapes of the stripes consistent, no methods are known to exist to either verify that the stripes achieve required specifications or to address the PES issues that result from nonlinear servo stripe configurations.
As mentioned, there are several position errors associated with servo stripes beyond those induced by the actual shape of the stripe. One such error is referred to as the offset error. Offset error occurs when the measured position differs from the actual position by a fixed amount. When this error arises from the tape itself each drive sees the same error and miss-positions data tracks by an equal amount. As long as the offset error is not too large the error does not present a problem. Another type of error is referred to as gain error. Gain error refers to a multiplication factor multiplied to the actual position. When the factor is not equal to one (1) the tracks can be placed to close together or to far apart. However, this error is easily analyzed by studying the difference in the reported positions between two widely space servo readers while reading the same servo band. Finally there is an error related to the non-linear relationship between the servo stripes measured position and the actual position that can cause the tracks to be placed to close together. This non-linear relation between the measured and actual position can result from the imperfect shape of the servo stripes themselves, i.e. not being an ideal trapezoid. This type of error is referred to as Differential Non-linearity and is the subject of the present application.